This invention relates to a process whereby sludge waste product can be converted into useful materials by employing a novel gravity pressure vessel.
Gravity pressure vessels have been employed in the treatment of waste streams. Specifically, this treatment has included pyrolysis, sub-critical wet oxidation, supercritical wet oxidation, and acid hydrolysis of waste streams. Where wet oxidation is employed, the treatment may include the complete oxidation of the materials within the waste stream or the controlled or restrained oxidation of these waste materials. In either wet oxidation method, oxygen gas (O2) has been employed as the reactant.
Use of O2 as a reactant within a gravity pressure vessel, however, can be disadvantageous. To begin with, O2 is not a readily available product or byproduct of waste treatment, and therefore the O2 must be obtained, i.e., it must be purchased, which can be costly. The use of O2 as a reactant is also problematic in that it presents safety hazards. For example, where the O2 is introduced into the gravity pressure vessel at or near the top of the reactor, the O2 must be introduced at a pressure that is greater than the pressure at the point of injection at the bottom of the reactor. Within some vessels, this pressure could be as high as 2,000-4,000 psi. Further, where the O2 is added at the top of the reactor, the O2 will heat to temperatures as great as 750xc2x0 F. as the O2 proceeds to the bottom of the reactor. At these temperatures and pressures, the O2 is extremely reactive and, therefore, in addition to presenting safety problems, may in fact destroy the reactor conduits that deliver the O2 to the bottom of the reactor.
Attempts at solving the problems associated with the use of O2 as a reactant have been proposed. In one instance, the prior art teaches constructing a pump station deep within the strata whereby O2 could be pumped into the reactor at the pressure at the point of entry. This procedure, of course, is costly and prone to mechanical failure.
Other attempts have included proposals to introduce oxygen into the waste stream as the waste stream proceeds down the reactor. This procedure, however, proved to be problematic because the oxidation reaction began as the fluid flowed to the bottom of the reactor and therefore the ability to achieve a peak reaction temperature was hindered. As a result, insufficient degradation of waste products occurred.
In still other applications of wet oxidation, hydrogen peroxide is used as a liquid form of oxygen. Hydrogen peroxide, is several times more expensive than oxygen gas under pressure and the pre-heating of the hydrogen peroxide causes it to decompose, making control of the net injection into a gravity pressure vessel problematic. The use of hydrogen peroxide as an oxidant in wet oxidation is thus limited to small capacity units that employ a pump and reactor tank that does not involve any significant pre-heating of the oxygen feed.
Although numerous problems have ben confronted in the wet oxidation of waste streams within gravity pressure vessels, wet oxidation treatment processes have significant potential for the efficient and environmentally sound treatment of waste streams. Accordingly, there remains a need to improve upon these processes, especially those processes.
In general the present invention provides a method for treating a waste stream, the method including the steps of providing a waste stream that includes waste solids suspended in water, feeding the waste stream to the top of a hydraulic downdraft column, conducting the waste stream to the bottom of the hydraulic downdraft column, conducting the waste steam to a first reaction zone, introducing nitrogen-containing oxides into the first reaction zone so that the waste stream is contacted with the nitrogen-containing oxides, conducting the waste stream up a hydraulic updraft column into a second reaction zone, where the second reaction zone is configured to provide sufficient time so that a reaction between the nitrogen-containing oxides and waste solids can take place and substantially consume the nitrogen-containing oxides, introducing oxygen gas into the waste steam after the nitrogen-containing oxides are substantially consumed, thereby providing a second reactant that reacts with waste solids suspended within the stream, conducting the stream to the top of the hydraulic updraft column.
The present invention also includes a wet oxidation process for treating a waste stream of the type that employs a gravity pressure vessel, where the improvement comprises providing oxides of nitrogen to the gravity pressure vessel as an oxidant.
The present invention advantageously overcomes problems associated with the prior art by employing oxides of nitrogen as an additional or alternative oxidant in the wet oxidation of waste streams. The oxides of nitrogen can be added to the reactor in the aqueous state, and by doing so only minimal pressures are required to introduce the oxidant into the reactor. Also, by employing nitrogen oxides, ammonia is provided as a byproduct of the wet oxidation reaction. This ammonia can be advantageously captured and employed as a useful product such as in the production of urea. Alternatively, the ammonia can be converted into oxides of nitrogen and recycled as a reactant in the wet oxidation process. Further, the use of nitrogen containing oxides as an oxidant has led to the unexpected discovery that sub-critical wet oxidation processes can be made more efficient because mass transfer limitations associated with O2 can be minimized or eliminated.